PHOENIX (AP) – Arizona Diamondbacks ace Brandon Webb experienced a setback Friday in his attempt to recover from shoulder problems that have sidelined him since his opening-day start.

Webb’s scheduled bullpen session was canceled when the right-hander felt tightness in the shoulder while playing catch. It was supposed to be his second bullpen workout. He felt fine after his first, 33-pitch session on Tuesday.

The plan had been for Webb to throw to hitters if Friday’s bullpen went well, then perhaps have him make a minor league start. Manager Bob Melvin said he didn’t know what the next move would be now until he talked to the team doctor.

“It’s obviously a little confusing because he felt good the last time out,” Melvin said before the Diamondbacks hosted the San Francisco Giants. “All the maintenance stuff leading up to (Friday) would suggest he felt good. Once he played catch there was just some tightness in the same spot and we didn’t want to push it any further than that.”

An MRI showed a small amount of fluid in the shoulder but no structural damage.

“I don’t want to look too much into it,” Melvin said of the setback. “Hopefully it’s just a hiccup. It slows him down some but we have a lot of season left and obviously we want to make sure we get this guy right. It’s just something we’re going to have to deal with.”

Webb went on the 15-day disabled list April 13, retroactive to April 7.

The 2006 NL Cy Young Award winner, who was 22-7 last season, was rocked for six runs in his April 6 start, getting a no-decision in a 9-8 victory over Colorado. He was scrubbed from his next start, then the decision was made to put him on the DL.

It’s just his second trip to the disabled list in his seven major league seasons. The other was in 2003. Webb has pitched more than 200 innings in each of his last five seasons.

“He’s a guy we’re going to be careful with,” Melvin said. “We’re not going to push him and say “See if you can throw through it and see if it loosens up.’ He’s too important for us to try to push through something through that.”

Right-hander Yusmeiro Petit – 0-1, 6.00 ERA in three appearances, two starts – will continue to take Webb’s place in the rotation.

AP-ES-04-24-09 1945EDT